No need to convert your books. As cybmole said, the books you buyed from amazon, you can download again from the kindle app.
If you do want use calibre: I don't use the kindle app, so I just guess here:
If you want to sideload your books with calibre:
Connect your tablet to your pc with usb
Open calibre
Click on connect/share in the calibre toolbar
select "connect to folder"
navigate to your tablet and to the kindle folder and click on "choose folder"
Now calibre will send books to that folder.
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